Copolymers of acrylic acid and acrylamide (PAM) were introduced for improvement of soil quality via aggregation of soil particles over 50 years ago (U.S. Pat. Nos. 2,625,529 and 2,652,380 to Hedrick and Mowry (1953) and U.S. Pat. No. 2,652,381 to Basdekis (1953)). Commercial use of PAM in agriculture started to grow in the mid-90's and has gradually increased to levels of several million pounds per year of annual application in the U.S. In current use, the copolymers are generally dissolved in irrigation water at doses of 2 to 10 ppm, which converts to about 2 to 5 lbs per acre.
Principal benefits include soil retention and water conservation. Soil is retained on the fields via the agglomerating action of PAM as it flocculates soil constituents into larger, adhering particles that settle out of the flow. Hence, the water runs clear rather than turbid down the furrow. Moreover, because the fine powders and platelets of the soil become agglomerated as part of the settled particles, the natural porosity of the soil along the furrow does not become clogged with these fines as occurs in untreated furrows, and the infiltration of water into the soil is improved. Thus, water is conserved on the field, rather than flowing over and off the field, carrying topsoil with it. Ancillary benefits include reduced loss of adsorbed nutrients, fertilizers, and treatment chemicals such as herbicides and pesticides from the soil. These benefits lead to overall improvements in yields and crop quality.
On the other hand, there are problems with the ongoing usage of PAM in agriculture. For example, the polymers are generally very high MW (e.g. 12 to 22 million Da), which makes them slow to dissolve in water and produces very high viscosity solutions. Preparation of aqueous solutions requires vigorous stirring for extended periods to prevent the formation of viscous, gelatinous clumps, which remain insoluble and tend to clog metering equipment and delivery systems. Handling issues of this nature have significantly hindered the adoption of PAM by many growers.
In addition, the base monomer, acrylamide, is a hazardous, reactive monomer that is a known neurotoxin and suspected carcinogen. Of course, effective steps are taken to ensure that residual levels of monomer in the polymer products are well within safety limits. Nonetheless, it is evident that there are several properties and features of the copolymers of acrylate and acrylamide that are not desirable. Consequently, alternative materials are sought that will function well in agglomerating soil particles, without the handling issues and perceived environmental problems associated with PAM.
Other flocculation technologies for which environmentally friendly agents are sought include control of dust emissions into the atmosphere from particulate surfaces, in locations such as rural roads, construction sites, temporary landing pads and strips, and outdoor arenas and facilities. Currently used dust control agents have also been based on copolymers of acrylate and acrylamides, as well as other vinyl monomers that are derived from feedstocks of natural gas and petrochemicals (see e.g. U.S. Pat. Nos. 4,801,635, 5,514,412, and 5,242,248). Similar materials are used for the very high volume commodity markets for water treatment, including industrial and municipal water treatment (U.S. Pat. No. 5,178,774), treatment of process waters during mining operations (U.S. Pat. Nos. 4,253,970, 4,839,060, and 6,042,732), and treatment of municipal sewage and agricultural wastes for clarification and removal of suspended solids (U.S. Pat. No. 5,776,350).
A particular area in which improved flocculants are sought is in the area of oil-sands mining and processing. In this process, as described further below, roughly three barrels of oily and bituminous-containing process water are produced per barrel of oil. This process water is eventually recycled into the steam generators, but it must first be clarified and separate from substantial amounts of suspended and emulsified oil and bitumen. Because of the high oily and bituminous content of the process waters, ranging roughly from 1% to 60% solids, and the elevated temperatures involved (95° C. or higher), it has been challenging to design effective water-treatment protocols that clarify the water and provide good separation of the aqueous and petrochemical phases. Current practice for clarification of oil-sands process water typically employs high levels (thousands of ppm or more) of inorganic salts and/or polycationic agents, and take several hours, yet still often result in incomplete separation. Improved methods and compositions for clarifying such process waters are therefore desired.